At the present time, optical, bright-field, stereoscopic microscopes are used in connection with medical surgical procedures, for example in the microsurgery of the eye. Such microscopes have two eyepieces so that the viewer may view the object to be magnified using both his eyes at the same time. The viewer sees two side-by-side, magnified images of the object and produces a three-dimensional ("3D") image in his mind.
In the conventional stereoscopic microscope the individual images, and the resulting 3-D image, are not inverted. The viewer sees an image which is, in effect, in the same position as the object.
However, in some surgical procedures, especially in inner eye surgery or "vitrectomy surgery" for the back area of the retina, an additional magnifying lens system is positioned between the stereoscopic microscope and the object, i.e. the eye being operated upon. That additional lens system may invert the image so that the surgeon sees a mirror image. This presents a difficult problem for the surgeon in eye-hand coordination. Also, such inverted images may be presented in certain digital diagnostic systems.
German Gebrauchsmuster G3902035.9 of May 11, 1989, discloses a stereoscopic optical microscope in which the two images are inverted by complex mirror and prism arrangements. Each of the images is reflected from a prism to a first mirror, then to a second mirror and then to another prism. Each time the image moves through an air gap, from the prism to the mirror or from one mirror to another, it necessarily loses at least 6% in light, and therefore in clarity. Consequently, each image is degraded at least 18% by the inversion prism-mirror system of the German Gebrauchsmuster.